Tc99m-tetrofosmin/MIBI and thallium-201 perfusion agents concentrate mainly in viable tumour cells, less so in inflammatory cells and almost negligibly in necrotic tissue. Although early images may demonstrate uptake in infection because of passive diffusion, this usually clears on delayed 2-4 hr images. Thallium, Tetrofosmin and MIBI do not cross the BBB unless the barrier is disrupted. Tetrofosmin and MIBI have better tumour-to-background ratio than Thallium, but they are taken up choroid plexus (can be blocked by pre-treatment with potassium perchlorate).